1. Field of the Invention
The present invention relates to a method and apparatus for truing grinding tools and to tools ground by them. More specifically, the invention relates to a diamond grinding tool, a truing method and apparatus therefor, and a magnetic head which is finished by grinding by this grinding tool.
2. Description of the Prior Art
FIG. 17(a) shows the structure of a metal bond diamond grinding tool before truing, FIG. 17(b) shows the structure of the metal bond diamond grinding tool after truing, and FIG. 18 is a schematic view of the cutting edge condition of abrasive grains of the diamond grinding tool before truing.
To control the run out V of a grinding tool and correct the shape of the grinding tool is generally referred to as truing and to remove the bond and project abrasive grains is referred to as dressing. In the case of a diamond grinding tool 3 before truing, as shown in FIGS. 17(a) and 18, the cutting edge height of diamond abrasive grains 1, that is, the height of the tips of the diamond abrasive grains 1 is not constant.
Conventional diamond grinding tool truing methods are a method for processing a target grinding tool using a GC tool (grinding tool with SiC) as a tool, a method for processing a dressing tool, and a method for melting bond by discharge. These methods are described in "Studies on Truing of Diamond Vitrified Wheels" (JSPE 52-02-1986) and Japanese Patent Laid-Open No. 1989-188266, such truing methods designed to eliminate the run out of a grinding tool by removing the bond which is founded on drop out of grain.
FIG. 19 is a schematic view of the surface condition of a diamond grinding tool which is trued by the conventional truing method.
According to the conventional truing method, bond 2' is removed by truing, and diamond abrasive grains 1' supported by this bond 2' are dropped out, and excessively projected diamond abrasive grains 1' are removed. However, according to this truing method, the hardness of abrasive grains which are a tool is lower than the hardness of diamond abrasive grains 1 to be processed and hence the diamond abrasive grains 1 cannot be ground.
For ultra precision grinding of brittle materials which attracts a great deal of attention recently, shape accuracy and surface roughness of the order of nanometers are aimed at. To accomplish the aims, a processing condition that the cutting depth of each abrasive grain is less than the critical cutting depth is required for processing. It is well known that when a brittle material is ground, it is processed in the brittle mode with cracks produced. However, it is found that by controlling the abrasive grain cutting depth to a minute value for processing, a brittle material can be processed in the ductile mode which produces no cracks in the same way as a metal. The boundary between the ductile mode and the brittle mode is referred to as a critical cutting depth (dc value), which is reported as about 0.1 .mu.m, though it depends on the material. For that purpose, it is necessary to control the cutting edge condition of a grinding tool such that the grinding tool run out is reduced to the order of submicrons or less and the abrasive grain cutting edge height is constant.
FIG. 20 is a perspective view of a magnetic head and FIG. 21 is an enlarged drawing of the S section shown in FIG. 20.
The accuracy of magnetic heads is improved recently and particularly high shape accuracy and reduction in micro,step formed by processing are required. For finishing an air bearing surface of a magnetic head 4 which is opposite to a recording medium shown in FIG. 20, lapping is used at present. However, lapping is a processing method on the basis of the principle of pressure copying and hence the edge part under a high processing pressure is apt to be processed early. This causes the edges to be blunt and no high shape accuracy can be obtained. Since lapping uses free abrasive grains, a magnetic film 7 (Vickers hardness Hv=200) shown in FIG. 21 is low in hardness compared with a substrate 5 (Hv=1300) and a protection film 6 (Hv=1000) and hence is processed early and a micro step A' is formed by processing. If the air bearing surface of the magnetic head 4 can be processed by grinding which is a processing method on the basis of the principle of motion copying, it is possible to process the air bearing surface with high shape accuracy compared with lapping and to reduce the micro step formed by processing to 0 in principle. However, a problem imposed when lapping is replaced with grinding is that the processed surface roughness by grinding is worse than the processed surface roughness by lapping. Therefore, to process the magnetic head by grinding with high shape accuracy and good processed surface roughness, the diamond grinding tool requires the following points.
They are that diamond abrasive grains are bonded with metal bond with high holding stiffness, and the grinding tool run cut is controlled to the order of submicrons, and the abrasive grain cutting edge height is made constant.
The aforementioned conventional truing method is a so-called truing method founded on drop out of grain for removing the abrasive grain bond by truing and dropping out abrasive grains supported by this bond. However, according to this truing method, the hardness of abrasive grains which are a tool is lower than the hardness of diamond abrasive grains to be processed and hence the diamond abrasive grains cannot be ground. Therefore, the roundness of the trued diamond grinding tool is affected by the diamond abrasive grain distribution accuracy (an index indicating whether diamond abrasive grains are uniformly dispersed on the inner surface participating in processing of the grinding tool) on the peripheral surface of the grinding tool and the diamond abrasive grain size. As shown by A and B in FIG. 7, there is a problem that when the abrasive grain size of the grinding tool is large, the grinding tool run out increases. Furthermore, there is another problem that even when the abrasive grain size is small such as several .mu.m, the grinding tool run out is rather large such as 1 .mu.m and cannot be controlled to the order of submicrons.
As mentioned above, for ultra precision grinding of brittle materials such as ceramics, a processing condition that the cutting depth of each abrasive grain of the grinding tool is less than the critical cutting depth is required for processing. For that purpose, it is necessary to control the grinding tool run out to the order of submicrons or less and to make the abrasive grain cutting edge height constant. However, the conventional truing method does not take account of making the abrasive grain cutting edge height constant. Therefore, for a grinding tool trued by the conventional truing method, there is a problem that the processing condition that the cutting depth of each abrasive grain is less than the critical cutting depth and the ductile mode which produces no cracks in brittle materials cannot be applied to ultra precision grinding of brittle materials.
Furthermore, to grind the surface of a magnetic head which is opposite to a recording medium, it is necessary to process it with high shape accuracy and with processed surface roughness which is similar to the roughness by lapping. To grind the air bearing surface of the magnetic head with processed surface roughness which is similar to the roughness by lapping, it is necessary to form a diamond grinding tool by bonding diamond abrasive grains with metal bond with high holding stiffness, to control the diamond grinding tool run out, and to make the diamond abrasive grain cutting edge height constant. However, the conventional truing method gives no consideration to that a hard diamond grinding tool itself is ground and the diamond abrasive grain cutting edge height is made constant. Therefore, there is a problem that a diamond grinding tool trued by the conventional truing method cannot be applied to grinding the surface of a magnetic head which is opposite to a recording medium.